I have an update on the Firefox checkbox issue. I changed to the default Fx3 theme and the screenshot is what I got.
I'm going to label this bug a Compiz bug rather than a Mac4Lin bug. I'll also see if I can find anything about it in the Compiz community.
EDIT: I just disabled the Mac4Lin GTK theme and the problem went away. I'm assuming it's something about how checkboxes are shaded, but now that we have a suspect we may be able to resolve the bug sooner.